1. Field of the Invention
The present invention relates to steel compositions and methods of processing that provide lightly-tempered martensitic microstructures with good combinations of strength and toughness.
2. State of the Art
Steels with lightly-tempered martensitic microstructures are finding increased utilization in a variety of highly stressed structural, machine and automotive components. Most work in this field has been generally concerned with the development of toughness through control over the base steel composition (i.e., inherent matrix toughness) or the content and dispersion of non-metallic inclusions (i.e., steel cleanness and inclusion shape control). There more recently has been an increasing awareness of the deleterious effects of smaller second-phase particles such as grain-refining precipitates on the toughness of tempered martensite, and two general approaches have been taken to affect improvements in the toughness of steels containing these precipitates: (i) refinement of the precipitates and (ii) substitution of a less detrimental precipitate species for one which adversely affects toughness.
The first method of improving toughness, which is the basis of a patent by Leap (U.S. Pat. No. 5,409,554, 1995), entails processing to refine grain-refining precipitates in high-strength steels. This method has been shown to provide improvements in toughness over a broad range of strength in a wide variety of base steel compositions containing aluminum, microalloying elements, aluminum in conjunction with any reasonable combination of microalloying elements, and nitrogen in concentrations representative of electric-furnace (EAF) steelmaking practices (M. J. Leap and J. C. Wingert, "Recent Advances in the Technology of Toughening Grain-Refined, High-Strength Steels," SAE International, Paper 961749, 1996 and M. J. Leap and J. C. Wingert, "Application of the AdvanTec Process for Improving the Toughness of Grain-Refined, High-Strength Steels," 38.sup.th Mechanical Working & Steel Processing Conference Proceedings, Iron & Steel Society, Inc., 1996). This refinement-based mechanism of toughening has also been shown to provide improvements in the low-temperature toughness of high-strength steels (M. J. Leap, J. C. Wingert, and C. A. Mozden, "Development of a Process for Toughening Grain-Refined, High-Strength Steels," Steel Forgings: Second Volume, ASTM STP 1259, American Society for Testing and Materials, 1997).
The second method of affecting toughness in high-strength electric arc furnace or "EAF" steels is based on the precipitation of TiN in preference to AIN in a steel with a nominal composition of 0.3% C., 0.65% Mn, 1.5% Si, 2.0% Cr, 0.4% Mo, 0.1% V, 0.06% Ti, &lt;0.03% Al, and 50-130 ppm N (J. E. McVicker, U.S. Pat. No. 5,131,965, 1992). Although the steels evaluated in this patent exhibit exceptional combinations of hardness and short-rod fracture toughness (i.e., stably-constrained ductile tearing resistance), both the impact toughness and plane-strain fracture toughness of this steel are comparable to other alloy steels containing refined dispersions of grain-refining precipitates. A similar methodology has been taken by Bobbert et al. (U.S. Pat. No. 5,458,704, 1995), where titanium is utilized as a gettering agent for nitrogen in boron-treated steels containing 0.25-0.32% C., 0.1-1.50% Mn, 0.05-0.75% Si, 0.9-2.0% Cr, 0.1-0.70% Mo, 1.2-4.5% Ni, 0.01-0.08% Al, &lt;0.015% P, &lt;0.005% S, and &lt;120 ppm N.
Notwithstanding the degradation in toughness resulting from the purposeful addition of grain-refining elements to high-strength steels, various investigators have noted the potentially deleterious effects of residual alloy carbides (i.e., iron/alloy carbides retained through the hardening heat treatment) on the toughness of tempered martensitic microstructures. For example, Thomas and Rao (U.S. Pat. Nos. 4,170,497 and 4,170,499, 1979), Sarikaya, Steinberg and Thomas (Metallurgical Transactions, vol. 13A, 1982, 2227-2237), and Ramesh, Kim and Thomas (Metallurgical Transactions, vol. 21A, 1990, 683-695) have indicated that the development of good toughness in high-strength steels requires the elimination of coarse alloy carbides from the microstructure, and towards this end, two-stage austenitization treatments have been designed to obviate this problem. General variations on the two-stage heat treatments comprise austenitization at 1100.degree. C. followed by either quenching and reaustenitization at a lower temperature (870.degree. C.) or quenching, tempering at 200.degree. C., and reaustenitization at a lower temperature. As explained in these references, the high-temperature austenitization treatment is utilized to dissolve coarse iron/alloy carbides while the second austenitization treatment is necessary for grain refinement. However, since these references do not contain data for steels subjected to hardening treatments at conventional temperatures (i.e., austenitization in the 800-850.degree. C. range), it is not possible to evaluate the effects of the double-austenitization treatment on toughness via the postulated mechanism. These results are further confounded by the results of Sarikaya, Steinberg and Thomas, where undissolved carbides were not found in a series of alloy steels after the application of either single-austenitization or double-austenitization treatments.
A review of these investigations suggests that while methods have been developed to improve the toughness of high-strength steels containing grain-refining elements, no efforts have specifically focused on the improvements in toughness that can result from the virtual elimination of grain-refining precipitates in lightly-tempered martensitic microstructures. Moreover, relative to the postulated effect of residual iron/alloy carbides on toughness, which has been the basis for the commercially impractical two-stage austenitization treatments, no effort has concentrated on (i) isolating the deleterious effects of residual iron/alloy carbides on toughness and (ii) developing methods to alleviate the degradation in toughness associated with the presence of these particles in lightly-tempered martensite.